To paraphrase a famous quote in the history of space exploration, ion engine technol- ogy has taken a giant leap forward with the decisive flight validation of the NSTAR thruster aboard the interplanetary spacecraft Deep Space 1. Because the NSTAR technology was voluntarily conservative while yet offering spectacular benefits on comet rendezvous or sample return, Mars, and a variety of other outer planet missions, because improvements in total engine impulse capability provide a considerable lever- age on mission performance, and finally because there exist candidate technologies with the promise of allowing significant engine lifetime improvements, the prospects for dramatically more ambitious space exploration missions now appear extremely
favorable.
Appendix A
Divergence of a Non-Neutralized Beam
The divergence angle for a non-neutralized beam of particles carrying a charge q and streaming at velocity v is the result of the action of the outward electric force qE, where E is the local electrostatic field induced by the space-charge, and the inward force qv×B produced by the azimuthal magnetic field B. The net (radial) force is then given by (see for example Ref. [196])
qE−qvB =
µ qIb
2π²0Rv
¶ ³1−²0µ0v2´ (A.1)
where Ib the total beam current, ²0 and µ0 are respectively the permittivity and permeability of vacuum, and R is the beam radius. Although the beam divergence is not linear with source-to-target distance d, the angle subtended by the beam spot viewed from the source can be given by
δ = arcsin
1
2d(qE−qvB)
Ãd v
!2
(A.2)
Thus, for a fully non-neutralized beam of 100-eV, singly-charged xenon ions with current density 15 nA/mm2 and initial radius 2 mm, the spot on a target located 1 cm away from the source will subtend an angle of ≈2◦ from the source.
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